'The Law and the Prophets'
Our Lord Jesus was born a Jew and He must be a devout Jew otherwise He would be counted as a sinner or hypocrite. He must abide to Mosiac Law and the Prophets. How difficult it was for our Lord Jesus to fulfill and obsolete the Law and then set up the New Covenant of grace by His blood.
Word was made Flesh
Our Lord was the Word made flesh, so His commandments, words, teachings, etc. are God's doctrine.
John 7:16 Jesus answered them and said, "My doctrine is not Mine, but His who sent Me.
Full of Grace and Truth
John 1:14 And the Word became flesh and dwelt among us, and we beheld His glory, the glory as of the only begotten of the Father, full of grace and truth.
Our Lord came with grace and truth to fulfill the Prophets, for the throne of God is the mercy seat.
Hos 6:6 For I desire mercy and not sacrifice, And the knowledge of God more than burnt offerings.
Fulfilled the Law or the Prophets
Mat 5:17 "Do not think that I came to destroy the Law or the Prophets. I did not come to destroy but to fulfill.
As long as He was under the Law as a Jew, He must abide in the Law. So before His death to obsolete the Law, you cannot find Him teaching the Jew to break any Law including sacrifices, Jewish feasts, etc.
He only could teach them they commandments which fulfilled the Law and not break the Law.
Examples are found in the Sermon on the Mount (Mat 5-7)
Angry without a Cause
Mat 5:21-22 "You have heard that it was said to those of old, 'You shall not murder, and whoever murders will be in danger of the judgment.' But I say to you that whoever is angry with his brother without a cause shall be in danger of the judgment. And whoever says to his brother, 'Raca!' shall be in danger of the council. But whoever says, 'You fool!' shall be in danger of hell fire.
Our Lord told them that the Law condemned murderers, but He taught them that getting angry with a brethren without a reason was as good as killing him.
Lusting after a woman is Adultery
Mat 5:27-28 "You have heard that it was said to those of old, 'You shall not commit adultery.' But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart."
Our Lord told them that the Law condemned adultery, but He taught them that those looking at a woman with lust had already committed adultery in his heart.
Do not Swear at All
Mat 5:33-35 "Again you have heard that it was said to those of old, 'You shall not swear falsely, but shall perform your oaths to the Lord.' But I say to you, do not swear at all: neither by heaven, for it is God's throne; nor by the earth, for it is His footstool; nor by Jerusalem, for it is the city of the great King.
Love your Enemies
Mat 5:43-44 "You have heard that it was said, 'You shall love your neighbor and hate your enemy.' But I say to you, love your enemies, bless those who curse you, do good to those who hate you, and pray for those who spitefully use you and persecute you"
OT commandment- love your neighbour but hate your enemies
NT commandment - love your enemies
Commandments lead to Holiness
1 Tim 1:5 Now the purpose of the commandment is love from a pure heart, from a good conscience, and from sincere faith
More Grace from our Lord
Jam 4:6 But He gives more grace. Therefore He says: "God resists the proud, But gives grace to the humble."
Brethren, when we are unable to follow His commandments, we must be humble before Him and confess our weaknesses, and He will grant us more grace by empowering us.